The method relates to a method of manufacturing a pellistor.
Pellistors are catalytic oxidation sensors which measure concentrations of combustible gases in air up to the lower explosive limit. The sensors are matched in pairs of elements, each conventionally comprising a platinum wire coil embedded within a catalytic bead. An active detector element oxidises combustible gases, while an inert reference element, named the compensator, compensates for changes in ambient conditions. The coil serves two purposes. Firstly, in the detector and compensator it is used to heat the bead electrically to its operating temperature (about 500xc2x0 C.); and secondly, for the detector, it is also used to detect changes in temperature produced by the oxidation of the flammable gas.
A Wheatstone Bridge circuit is used to measure the concentration of combustible gas in air. The Bridge is balanced by a variable resistance, with both elements at their operating temperature; an out-of-balance signal is produced when a combustible gas is detected. The signal is proportional to the concentration of combustible gas.
More recently, attempts have been made to manufacture planar pellistor arrangements in which the conventional coil is replaced by a planar electrode on an inert substrate such as silicon. Typically, the silicon is micro machined to provide a thin membrane on which the heater is deposited. This is described in xe2x80x9cMicrosensors, Principles and Applicationsxe2x80x9d, Julian W. Gardner, pages 242-243, published by John Wiley and Sons. Potentially, planar arrangements could provide a number of important benefits. These are as follows:
Lower power: Conventional coil pellistors can only be produced with power consumption down to about 100 mW per element (i.e. 200 mW per pair). The platinum wire diameter to wind coils for these power levels is about 10 xcexcm. This is the practical lower limit at which it is possible to work, due to a combination of visibility and other handling difficulties. Micro machining is capable of fabrication down to much smaller dimensions and device powers of between ⅓ to ⅔ those of conventional devices have been achieved (e.g. a device manufactured by Microsens S.A. has a specification of 60 mW (25 mA at 2.4 volts) for a detector/compensator pair). The ability to integrate two or more elements (i.e. a detector/compensator pair) in very close proximity can also provide economies in power consumption.
Volume production: Conventional coil pellistor manufacture is an inherently labour intensive operation and is fundamentally not amenable to scale up for volume production. Micro machining, by its very nature, is a volume production method of fabrication and less suited to small-scale operation due to the high capital investment required. Depending on the exact technique employed, however, there are numerous contractors able to undertake production runs of different sizes. In principle, micro machined devices could be manufactured more consistently and cost effectively than conventional devices for a volume market. Even for relatively low volume industrial markets there would be a manufacturing advantage to the micro machined products which could produce more consistent quality devices with an automated method of fabrication and might offset any increased production cost associated with relatively small batch sizes.
Improved sensitivity: Conventional coil technology limits the type of material which can be employed due to the mechanical requirements placed upon the wire. In micro machined devices, it is common to employ encapsulated heaters, hence allowing the possibility of employing materials with more favourable properties. For example, metals or alloys with higher temperature coefficients of resistance than the normally employed Pt may offer improved resolution.
Pellistors have so far found applications in industrial areas to provide a warning of combustible gas accumulation to explosive levels, e.g. oil rigs, mines, sewers and other confined spaces. These markets are relatively small and suited to conventional pellistors. Other markets exist for lower cost devices, such as domestic applications which have so far been addressed by semiconductor devices. However, these suffer from well known shortcomings in performance, which severely limit their applicability.
The catalytic coatings produced on planar devices have usually been laid down onto the micro mechanical heater substrates with coating methods (such as vapour deposition or sputtering) which result in a relatively low surface area catalyst layer. This tends to produce devices whose catalytic activity is inherently poor and which have comparatively short operational lifetimes compared to conventional pellistors. This is particularly true when such devices are operated in environments containing materials which poison and/or inhibit the catalyst surface, e.g. silicone vapours, hydrogen sulphide. It is well known that the poison resistance in such devices is greatly enhanced by using high surface area catalysts which offer some redundancy of sites.
Attempts have been made to coat the micro machined planar heater substrates with conventional catalyst material mixes, but it is very difficult to do this accurately on areas with dimensions well below 1 mm, as is often required on micro machined devices. Furthermore, the micromachined heaters are unlikely to have sufficient mechanical strength to allow conventional methods (requiring contact with the substrate) to be employed, despite the fact that they may have excellent performance in response to thermal or mechanical shock. Additionally, the adhesion of the catalyst to the substrate is generally very poor. In extreme cases, the catalyst layer breaks away from the substrate resulting in total device failure, and/or heat transfer from the substrate to the catalyst is poor, resulting in higher power consumption to maintain the catalyst at its optimum operating temperature.
In accordance with the present invention, a method of manufacturing a pellistor comprises providing a porous catalyst layer on a heater by electrodepositing material from a mixture containing the catalyst and a structure-directing agent in an amount sufficient to form an homogenous lyotropic liquid crystalline phase in the mixture.
We have realised that it is possible to achieve very good porous catalyst layers having high surface areas using an electrodepositing technique. Although this is particularly suitable for use with planar pellistors and thus micro machined structures, the technique could also be applied to non-planar substrate geometries including conventional coil heaters.
We have found that the new catalyst layer can withstand the high temperatures associated with pellistor operation and is also durable and substantially poison resistant.
The process enables closely controlled porous catalyst layers to be laid down, if required in very small regions such as less than 100 xcexcm2, for example down to about 50 xcexcm2. Typical pore sizes are in the mesoporous range with internal diameters from 13 to 200 Angstroms, preferably 17 to 40 Angstroms.
It should be noted in particular that this method allows the amount and location of catalyst to be optimised in contrast to conventional pellistors where the bead is required to provide a support for the catalyst and introduces a significant heat sink thus requiring wasteful power input. The invention, in contrast, provides a substantially pure catalyst layer without any other material being present to act as a heat sink.
The material may be deposited onto the heater through a mask to provide even further control of the deposit area.
A particularly useful technique is described by Attard et al in xe2x80x9cMesoporous Platinum Films from Lyotropic Liquid Crystalline Phasesxe2x80x9d, Science, Vol. 278, 31 Oct. 31, 1997, pages 838-840.
The structure-directing agent is included in the mixture in order to impart an homogeneous lyotropic liquid crystalline phase to the mixture. The liquid crystalline phase is thought to function as a structure-directing medium or template for film deposition. By controlling the nanostructure of the lyotropic liquid crystalline phase, and electrodepositing, a film may be synthesised having a corresponding nanostructure. For example, films deposited from normal topology hexagonal phases will have a system of pores disposed on an hexagonal lattice, whereas films deposited from normal topology cubic phases will have a system of pores disposed in cubic topology. Similarly, films having lamellar nanostructures may be deposited from lamellar phases.
Accordingly, by exploiting the rich lyotropic polymorphism exhibited by liquid crystalline phases, precise control over the structure of the films is achieved, enabling the synthesis of well-defined porous films having a long range spatially and orientationally periodic distribution of uniformly sized pores.
Any suitable amphiphilic organic compound or compounds capable of forming an homogeneous lyotropic liquid crystalline phase may be used as the structure-directing agent, either low molar mass or polymeric. These may include compounds sometimes referred to as organic directing agents. In order to provide the necessary homogeneous liquid crystalline phase, the amphiphilic compound will generally be used at a high concentration, typically at least about 10% by weight, preferably at least 20% by weight, and more preferably at least 30% by weight, based on the total weight of the solvent and amphiphilic compound.
Suitable compounds include organic surfactant compounds of the formula RQ wherein R represents a linear or branched alkyl, aryl, aralkyl or alkylaryl group having from 6 to about 60 carbon atoms, preferably from 12 to 18 carbon atoms, and Q represents a group selected from: [O(CH2)m]n) OH wherein m is an integer from 1 to about 4 and preferably m is 2, and n is an integer from 2 to about 60, preferably from 4 to 8; nitrogen bonded to at least one group selected from alkyl having at least four carbon atoms, aryl, aralkyl, and alkylaryl; and phosphorus or sulphur bonded to at least two oxygen atoms.
Other suitable structure-directing agents include monoglycerides, phospholipids and glycolipids.
Preferably, non-ionic surfactants such as octaethylene glycol monododecyl ether (C12EO8, wherein EO represents ethylene oxide) and octaethylene glycol monohexadecyl ether (C16EO8) are used as structure-directing agents.
Further details of preferred aspects of this method are described in WO 99/00536, the content of which is included herein by reference.
Any conventional catalyst can be used, typical examples including palladium, platinum, iridium and rhodium. In addition, mixtures of two or more of these could be used while one or more could be codeposited together with a support such as alumina or silica.
In the case of a planar electrode, this may be in a serpentine form in order to increase the length of the electrode within a predefined area.
The ability to localise the catalyst in the regions where the heater is known to be operating at maximum efficiency optimises the sensitivity obtained per unit power input.
In general, prior to the electrodepositing step, the method comprises providing an electrode on the heater structure which contacts the mixture during the electrodepositing process. This enables the region of deposit to be controlled and also separates the components. involved with the electrodepositing step from the heater structure. However, it may be possible in some circumstances to use the conductor forming the heater as one of the electrodes which is used during the electrodepositing step.